1. Field of the Invention
The present invention relates to a display device, and more particularly, to a display device to which a common voltage is applied effectively and a method of making the same.
2. Description of the Related Art
Organic light-emitting diode (“OLED”) display devices are gaining popularity due to their advantageous features such as lower voltage drive, light weight, thinness, wide viewing angle, and high speed response as compared to other flat panel displays. OLED displays are classified into either passive matrix type or active matrix type. The passive matrix type has a simple structure, however power consumption is rapidly increased with display area and resolution. Thus, the passive matrix type of OLED display is primarily employed in small-sized applications. In contrast, the active matrix type display has the drawback of a complex manufacturing process, but has the advantage that it can be implemented with a wide screen and a high resolution.
In the active matrix type OLED thin film transistors (“TFTs”) are connected with each pixel region and they control the light emission of an organic light-emitting layer of each respective pixel region. Pixel electrodes are also located in each pixel region, each of which is electrically separated from other adjacent pixel electrodes for independent drive. In addition, a bank, which stands higher than the pixel electrodes, is formed between the pixel regions and serves to prevent short circuits between the pixel electrodes and further serves as an insulator between the pixel regions. A hole injecting layer and an organic light-emitting layer are sequentially formed on the pixel electrode between the banks separating the pixel regions. A common electrode is then formed on the organic light-emitting layer.
OLED displays are further classified into bottom emission type and top emission type according to a direction of light generated from the organic light-emitting layer.
The bottom emission type of OLED display directs the light from the organic light-emitting layer in a direction toward the TFT. The process of manufacturing the bottom emission type is well established, but has an inherent problem in that its aperture ratio is decreased due to the TFT and the wiring requirements. In particular, the TFT using amorphous silicon is typically larger to provide sufficient current flow from source to drain due to its low electron mobility, and the aperture ratio of such an OLED, which generally employs two or more TFTs, is even further decreased.
The top emission type of OLED display directs the light from the organic light-emitting layer through the common electrode to the outside. Accordingly, the top emission type may be constructed with a larger aperture ratio. In the top emission type, the common electrode should be transparent. Currently, the transparent common electrode is made through a thin metal layer deposition method (e.g., sputtering) using materials such as indium tin oxide (ITO) or indium zinc oxide (IZO). However, when constructing large sized displays the current method of manufacturing produces a common electrode with too much resistance. This has been an impediment to creating larger displays.